Polyurethanes are employed in many application fields, such as furniture, mattresses, transport, electrical, construction and industrial insulation. To achieve the high flame retardancy requirements demanded of materials for, inter alia, automotive, railway and aeroplane interiors and also for buildings insulation, polyurethanes and in particular polyurethane foams generally require treatment with flame retardants. To this end, a great many different flame retardants are already known and commercially available. However, there are often appreciable technical issues and/or toxicological concerns surrounding their use.
When, for instance, solid flame retardants are used, for example melamine, ammonium polyphosphate or ammonium sulphate, sedimentation or aggregation gives rise to metering problems which often necessitate technical modifications to the foaming equipment, i.e. costly and inconvenient revamping and rejigging.
It is true that the commonly used chloroalkyl phosphates tris(chloroethyl) phosphate, tris(chloroisopropyl) phosphate and tris(dichloroisopropyl) phosphate are readily meterable liquids. However, a recent but increasingly common requirement of open-cell flexible polyurethane foam systems for automotive interiors is that the gaseous emissions (volatile organic compounds, VOCs) and especially the condensable emissions (fogging) from these foams shall not exceed low limits. The liquids referred to above no longer meet these requirements owing to their excessive volatility.
Fogging refers to the undesired condensation of evaporated volatile constituents from the motor vehicle interior on glass panes, in particular on the windscreen. This phenomenon is quantifiable according to DIN 75 201 B. The automotive industry typically requires that the fogging condensate as determined by the DIN 75201 B method shall be less than 1 mg.
Furthermore, halogen-free flame retardants are preferred from ecotoxicological aspects and also by reason of ameliorated fire side-effects regarding smoke gas density and smoke gas toxicity. Halogen-free flame retardants may also be of particular interest for performance reasons. For instance, severe corrosion is observed on the plant components used for flame lamination of polyurethane foams when halogenated flame retardants are used. This is attributable to the emissions of hydrohalic acid which arise during the flame lamination of halogen-containing polyurethane foams.
Flame lamination refers to a process for bonding textiles and foams together wherein one side of a foam sheet is incipiently melted by means of a flame and immediately thereafter pressed together with a textile web.
The halogen-free liquid flame retardant systems hitherto disclosed, for example triethyl phosphate or other alkyl or aryl phosphates, such as diphenyl cresyl phosphate for example, provide only inadequate compliance with the abovementioned requirements for low levels of VOCs or low levels of fogging, or exhibit inadequate flame retardancy.
Hydroxyl-containing poly(alkylene phosphates) provide solutions in the sense of low fogging contributions. These substances contain alcoholic hydroxyl groups which during production of the polyurethane foams react with the polyisocyanates employed and thus ensure that the flame retardants are securely embedded in the polymer matrix. Hydroxyl-containing poly(alkylene phosphates) and processes for the production thereof are prior art, for example from DE-A 20 36 587, DE-A 20 36 595 B1, EP-A 0 658 561, EP-A 0 658 580 or EP-A 0 771 810.
The hydroxyl-containing poly(alkylene phosphates) described in the cited prior art have disadvantages. Their production requires the use of the phosphorus-containing raw material phosphorus pentoxide. Phosphorus pentoxide is a highly corrosive, extremely hygroscopic solid which is difficult and dangerous to handle. A synthesis of hydroxyl-containing poly(alkylene phosphates) using raw materials that are easier to handle is therefore desirable.
In addition, the prior art production processes afford the hydroxyl-containing poly(alkylene phosphates) in impure form. The production of said poly(alkylene phosphates) forms undesired phosphorus-containing compounds, in particular five-membered, cyclic phosphates. E. D. Well, R. B. Fearing, F. Jaffe, J. Fire Retardant Chem., 1982, 9, 39 reports that the presence of these five-membered, cyclic phosphates may be readily detected by 31P NMR spectroscopy since their resonance signal appears at a chemical shift of 17-18 ppm and thus stands out clearly from the signals of the main products (poly(alkylene phosphates)). According to Weil et al. the presence of these five-membered, cyclic phosphates results in undesired susceptibility to hydrolysis and in acid formation. Furthermore, poly(alkylene phosphates) having a high content of such undesired phosphorus-containing compounds, in particular of five-membered, cyclic phosphates, also contain more volatile components and thus also lead to elevated amounts of fogging condensate.
The problem of the occurrence of undesired phosphorus-containing by-products, in particular five-membered, cyclic phosphates, during production of phosphorus-containing oligomeric condensation products has been known about for a long time and there have been numerous proposals for solving it as is evidenced by U.S. Pat. No. 3,891,727, U.S. Pat. No. 3,959,415, U.S. Pat. No. 3,959,414, U.S. Pat. No. 4,012,463 and EP-A 0 448 159. In all of these proposed solutions the avoidance or reduction in the levels of such impurities involves increased cost and complexity, for example in the form of downstream purification steps, in the synthesis of these phosphorus-containing oligomeric condensation products.
An improved route to hydroxyl-containing poly(alkylene phosphates) having a relatively low content of undesired phosphorous-containing by-products, in particular five membered, cyclic phosphates, is therefore desirable. The present invention has for its object the provision of hydroxyl-containing poly(alkylene phosphates) which overcome the cited disadvantages of the prior art.